We have wave soldering machines with SnPb.... the problem is they want to shut off the exhaust fan when there is no production even if the entire machine was not switch Off and the pots are maintain the 250°C. Can someone give any idea on the implications of this action?

Simple answer is no you cannot do it and why would you?.. is it a cost saving thing? As Dave F correctly points out you have a fair amount of noxious fumes to handle that aren't getting exhausted if you switch of the extraction so they will go to your plant. This will also over heat and oxides will bake on to your heaters as well. (did I mention the sky will fall in?.. no?.. ok it wont but the roof could) Most wave machines have idle settings which would let you reduce the pot temp down which would be a bigger saving that switching off the fume extraction